The 43-year-old ex-boyfriend showed up at the University Avenue unit in the Bronx around 4:30 a.m. to talk to the mother, police told WPIX. The two argued in the hallway, then he followed her into the apartment.

The man, who police didn't immediately identify Monday, then began beating the 37-year-old woman, officers said. Her 18-year-old son heard what was happening and rushed out of his bedroom.

The teen allegedly fought with his mom's ex-boyfriend, pulling him off of his mother before choking the man to death, police said.

Police sources said the mother, who had briefly passed out, woke up to the sight of her son standing over her ex-boyfriend.

The dead man has 26 prior arrests, two for domestic violence against the teenager's mother, sources told WPIX.

Police took the teen into custody. No charges have been filed as of Monday evening.